Anthrax is one of the best known agents of biological warfare - and possibly one of the most feared.

The bacterium occurs naturally, in low levels, in some animals, but when it is inhaled by humans in the form of spores it is deadly.

The killing power of anthrax was demonstrated by British scientists during the Second World War when it was released on a tiny Scottish island to wipe out a flock of sheep.

The island, Gruinard, just off the mainland, in Gruinard Bay, half way between Ullapool and Gairloch in the Highlands, was so contaminated that it was deemed out-of-bounds for almost 50 years.

Gruinard Island is in the Scottish Highlands

The 1942 test was sanctioned amid fears the Germans might attack the UK with biological or chemical weapons.

Anthrax can be contracted by skin contact, ingestion or inhalation, but it is through inhalation that it is at its most deadly and proves to be fatal in about 95% of cases, even with medical treatment.

Experts on biological weapons have suggested that 100kg of anthrax sprayed on a major city could kill more than 3m people.

When anthrax spores are inhaled death usually takes around seven days and will be as a result of symptoms like internal bleeding, blood poisoning or even meningitis.

Initial symptoms after inhalation might include mild fever, malaise, fatigue, coughing and, occasionally a feeling of pressure on the chest.

Decontamination project

It was previously not uncommon for animal workers to become infected with anthrax through skin contact - it was called woolsorter's disease at one point - when a boil would appear which would eventually form a black centre.

When scientists experimented with anthrax on Gruinard Island a film was made of their work and it remained classified until 1997.

Sheep were taken to an open field, secured in wooden frames, and exposed to a bomb that scattered the spores. The sheep started dying three days later.

Despite attempts to disinfect Gruinard Island, the spores left by the experiments kept the island in quarantine for 48 years.

The final WW II report on the Gruinard Island tests suggested anthrax could be used to render cities uninhabitable "for generations".

I would not go walking on Gruinard. If anthrax is still active at Soutra, there is no reason to suppose it has not survived on more recent sites. It is a very resilient and deadly bacterium

Dr Brian MoffatArchaeologist

In 1986 an English company was paid half a million pounds to decontaminate the 520-acre island by soaking the ground in 280 tonnes of formaldehyde diluted in 2000 tonnes of seawater.

Topsoil was also removed in sealed containers.

To prove that the clean-up was successful a flock of sheep was allowed to graze the island at the behest of an independent watchdog set up by the Ministry of Defence.

On 24 April, 1990, the then junior Defence Minister, Michael Neubert, made the half-mile journey from the mainland to declare Gruinard safe by removing its red warning sign.

But at that time the Glasgow Herald newspaper reported that a leading archaeologist was unconvinced by official assurances the land was safe.

Vaccine

Dr Brian Moffat, archaeological director of an excavation of a medieval hospital near Edinburgh, said his team had encountered buried anthrax spores which had survived for hundreds of years.

He told the paper: "I would not go walking on Gruinard. If anthrax is still active at Soutra, there is no reason to suppose it has not survived on more recent sites. It is a very resilient and deadly bacterium."

However, that was not the end of Britain's interest in anthrax because earlier this month it was announced that a team led by a Scottish scientist had produced a vaccine.

It was manufactured after two years work at Porton Down, the government's chemical and biological warfare research centre.

Sir William Stewart, who led the research group, said the vaccine should act as a safeguard for the future.